In the last few years, the use of plastic containers instead of glass containers has taken place on an increasingly large scale. As a consequence, the number of used plastic containers, such as bottles, also increases substantially. Although in principle, the used plastic bottles are indeed suitable, like glass bottles, for being refilled a number of times, the number of plastic bottles that are no longer suitable for being filled and sold again, and that should hence be considered waste material, increases as well. Usually, however, the material which the plastic bottles consist of is in fact suitable for being reused for manufacturing plastic bottles.
However, this involves the problem that it is often required that the reused material in the final product manufactured by means thereof, do not come into contact with the contents of the containers.
Sometimes, it is also desired for other reasons to provide a core layer of a different material in the wall, or a part thereof, of a plastic container. Such a core layer can for instance serve as a barrier layer to render the wall of the container to be formed impermeable to specific substances. For instance, the core layer can have special water-impermeable or oxygen-impermeable properties to prevent diffusion of water or oxygen through the container wall. Depending on the properties desired, such a core layer can consist of virgin or reused material.
A problem that may be involved in the use of a core layer is that the bond between the core layer and the material layers enveloping the core layer on the inside and the outside of the casing proves to be insufficient. In that case, the layers may come loose from each other, which may manifest itself when the preforms are inflated into bottles or when the preforms cool down, or during the use of the bottles, as a consequence of which the container may become useless.